1. Field of the Invention
The present invention relates to a semiconductor laser device used in such fields as optical communication and optical information processing.
2. Description of the Related Art
Semiconductor lasers which are capable of emitting laser light having wavelengths in the visible region have such applications as a light source for optical information processing and a light source for optical measurement, and have been gaining importance recently. And much effort has been made to develop various semiconductor materials suitable for such semiconductor laser devices. Among all possibilities, materials based on (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P have become a focus of attention because they are capable of lattice matching with GaAs which makes a good substrate, and are capable of generating laser light in a wavelength band from 0.68 .mu.m to 0.56 .mu.m by changing the Al mole fraction x. The wavelength of the laser light can be shortened by simply increasing the Al mole fraction in the active layer of the semiconductor laser device.
A transverse mode control type semiconductor laser device (for the emission of red light) will be described below with reference to FIG. 16.
The semiconductor laser device has an n-GaAs substrate 1 and a multi-layered structure formed on the substrate 1 as shown in FIG. 16. The multi-layered structure includes an n-GaAs buffer layer 2 formed on the top surface of the substrate 1, an n-(Al.sub.0.6 Ga.sub.0.4).sub.0.51 In.sub.0.49 P cladding layer 3, a (Al.sub.0.15 Ga.sub.0.85).sub.0.51 In.sub.0.49 P active layer 4, a p-(Al.sub.0.6 Ga.sub.0.4).sub.0.51 In.sub.0.49 P cladding layer 5, a p-Ga.sub.0.51 In.sub.0.49 P layer 7, an n-GaAs current block layer 8 and a p-GaAs cap layer 9. Bottom surface of the substrate 1 and top surface of the cap layer 9 are provided with an n-electrode 10 and a p-electrode 11, respectively.
Such a semiconductor laser device is capable of confining the current within a relatively narrow area by means of the n-GaAs current block layer 8. When etching the p-(Al.sub.0.6 Ga.sub.0.4).sub.0.51 In.sub.0.49 P cladding layer 5 in the form of a trapezoid during the manufacturing process, an effective refractive index difference which satisfies the conditions for single transverse mode emission can be easily provided by optimizing the height and width of the trapezoid. As a result, it is made possible to confine the light effectively within the narrow area of the active layer 4 at the base of the trapezoid.
However, when the amount of Al added to the active layer is increased, namely value of the Al mole fraction x is increased for the purpose of obtaining shorter wavelength of the laser emission, a difference .DELTA. Eg in energy band gaps between the (Al.sub.0.6 Ga.sub.0.4).sub.0.51 In.sub.0.49 P cladding layer and the active layer decreases. As a result, carriers injected into the active layer 4 for the laser emission can easily cross the barrier between the cladding layer 3, 5 and the active layer 4, thereby to reach the cladding layer 3, 5. This phenomenon is generally called a carrier overflow. The carrier overflow increases the number of carriers which do not contribute to the laser emission and deteriorates the characteristic of the semiconductor laser device. More specifically, the threshold current for laser emission increases to make laser emission at high temperatures difficult. The carrier overflow is more conspicuous, among the carriers, with electrons having less effective mass.
A semiconductor laser device has a limitation in emitting at shorter wavelengths, due to the restriction by the carrier overflow as described above. Continuous wave (CW) laser emission is limited to wavelengths not shorter than about 630 nm. Characteristics of semiconductor laser devices ( wavelength: 630 nm ) are not satisfactory, since a maximum CW operation temperature is about 40.degree. C. and maximum optical output power is as low as about 3 mW.
For the purpose of suppressing the electron overflow described above, a structure called a multi-quantum barrier (MQB) has been proposed (for example, the Japanese Journal of Applied Physics 29 (1990) pp. L1977-L1980). A prior art semiconductor laser device having a multi-quantum barrier will be described below with reference to FIG. 17 and FIG. 18. The semiconductor laser device has a multi-quantum barrier 13 comprising a p-(Al.sub.0.6 Ga.sub.0.4).sub.0.51 In.sub.0.49 P barrier layer 20 having a relatively broad band gap with 6-atom thickness (17 angstroms) and a (Al.sub.0.15 Ga.sub.0.85).sub.0.51 In.sub.0.49 P layer 21 having a relatively narrow band gap with 4-atom thickness (11.5 angstroms) ten times on one another between a p-(Al.sub.0.6 Ga.sub.0.4).sub.0.51 In.sub.0.49 P cladding layer 5 and a (Al.sub.0.15 Ga.sub.0.85).sub.0.51 In.sub.0.49 P active layer 4. Except for the multi-quantum barrier 13, the structure of this semiconductor laser device is similar to that of the semiconductor laser device shown in FIG. 16.
The multi-quantum barrier 13 is provided in order to reflect electrons (electron waves) toward the active layer 4 thereby to suppress the electron overflow. FIG. 19 shows the change in the electron-wave reflectivity with the electron energy in this laser device. Electrons encounter a barrier about twice higher than in a conventional laser device having no multi-quantum barrier. Because the electron overflow can be suppressed, a low threshold current and high temperature stability can be attained.
However, when one layer has a thickness deviation of a magnitude of a 2-atom layer or greater among the ten p-(Al.sub.0.6 Ga.sub.0.4).sub.0.51 In.sub.0.49 P barrier layer 20 of the multi-quantum barrier layer 13, the electron-wave reflectivity lowers at a position where the electron has a certain low energy. FIG. 20 shows the change of electron-wave reflectivity with the electron energy when the thickness of the sixth p-(Al.sub.0.6 Ga.sub.0.4).sub.0.51 In.sub.0.49 P barrier layer 20 decreases by the thickness of a 2-atom layer (5.66 angstroms). There is a drop in the reflectivity at an energy of 101 meV. A barrier height for the electron is determined by the minimum energy of electrons which can pass the barrier. Thus the barrier loses its barrier effect on the electrons when the thickness changes by the thickness of at least 2-atom layer. As a result, it becomes difficult to obtain a semiconductor laser device having good characteristics.
In the semiconductor laser device of the prior art described above, AlGaInP mixed crystal which includes aluminum (Al) is used as the material of the active layer in order to obtain shorter wavelength laser emission. However, because aluminum has a strong tendency of oxidation, increasing the amount of aluminum induces crystal defects leading to a gross decrease in the laser emission efficiency. Use of a multi-quantum well structure for the active layer eliminates such a problem, because the quality can be stabilized and emission at a short wavelength can be obtained even when GaInP which can be manufactured easily is used.
A multi-quantum well (MQW) laser of the prior art operating in 630 nm band will be described below.
FIG. 26A shows the energy band diagram of a typical multi-quantum well laser operating in 630 nm band, and FIG. 26B shows the energy band diagram of a separate confinement type multi-quantum well (SCH-MQW) laser. In the example of FIG. 26A, a multi-quantum well layer 234 made by laminating well layers 241 and barrier layers 242 alternately is interposed between a first cladding layer 232 and a second cladding layer 235. In the example of FIG. 26B, an optical guide layer 233 is further provided between the multi-quantum well layer 234 and the cladding layers 232 and 235.
In either example, ground level 236 of electron is set higher in the energy level than the edge of the conduction band, and the ground level 237 of hole is set higher in the energy level than the edge of the valence band. The differences (.DELTA. EC, .DELTA. EV) in the energy level between the ground levels 236, 237 and the band edges of the cladding layers 232, 235 are especially small with respect to the conduction band. This means that confinement of the carriers (electrons in particular) within the active layer is weak, resulting in leakage current due to massive diffusion of electrons into the p-cladding layer. That is, electrons supplied from the n-cladding layer cannot be sufficiently confined within the quantum well layer 241 leading to an overflow from the quantum well layer 241 to diffuse in the form of 3-dimensional.
Therefore such problems occur as electrons which do not contribute to the laser emission increase, threshold current for the laser emission increases or the threshold current heavily depends on the temperature. Although both examples of FIG. 26A and 26B have the problems roughly described above, the example of FIG. 26B has the optical guide layer 233, unlike the example of FIG. 26A, wherein light is confined. This makes it possible for the light emitted in the quantum well to effectively contribute to the stimulated emission, thereby reducing the threshold current of the laser. The leakage current described above can also be reduced, because the carrier density during laser emission is decreased.
However, even with the separate confinement type multi-quantum well structure (SCH structure) shown in FIG. 26B, the threshold current is still higher and the stability against temperature change is lower than in the case of other semiconductor laser devices. Thus reliability of the device is low and its application is limited. This is because of the significant leakage current due to the use of materials having relatively broad energy gap, and that the potential barrier of the quantum well (difference in the energy level between the ground level and the conduction band of the barrier layer) which leads to insufficient quantum effect.
FIG. 27 shows 2-dimensional state density of electron in the separate confinement type multi-quantum well structure wherein a multi-quantum well layer having well width of 3 nm, barrier width 5 nm and five wells is interposed between optical guide layers of thickness 65 nm. This figure shows that the state density changes with the energy in fine stair steps, because electrons cannot be completely confined within the wells so that electron wave packets are coupled with each other in the multi-quantum well layer. The figure shows that the state density shows a steep increase in the energy region beyond about 0.114 eV. This energy level corresponds just to the height of the potential barrier, indicating that electrons having energies beyond the barrier cannot be confined within the wells.
Consequently, no significant quantum effect can be obtained in this region because electrons overflow from the quantum well 241 as shown in FIG. 26B, causing the emission efficiency to decrease. In this structure, because the quasi Fermi level during laser emission exceeds 0.1 eV, a fairly large number of electrons can exist in this region while only the electrons around the ground level can contribute to the laser emission, thus the high-energy electrons become ineffective.
These high-energy electrons recombine with holes to cause ineffective current and increase the threshold current of the laser.
Due to the mechanism described above, only unsatisfactory characteristics have been obtained from the separate confinement type multi-quantum well laser of the prior art operating at short wavelengths. Also in a semiconductor laser device operating at a long wavelength-such as that based on InGaAsP/InP, for example, as well as in short wavelength lasers, population of electrons in the optical guide layer increases as the width of quantum well becomes very small, thereby decreasing the efficiency of the laser similarly to the cases described above. A single quantum well semiconductor laser device having only one well layer also has problems similar to those described above.